The invention relates to a solar receiver for a solar thermal power plant having trough-shaped concentrators for focusing incident solar radiation onto a solar receiver.
In solar thermal power plants, the incident solar radiation is amplified multiple times by being focused by concentrators onto a solar receiver absorbing the concentrated solar radiation and releasing it to a heat carrier medium, such as air or water. The heated heat carrier medium is then put to use, for example by being provided to a turbine for driving a generator.
In solar thermal power plants having trough-shaped concentrators, the solar receiver usually is tubular and arranged concentrically to the concentrator, which is mostly provided in the shape of a parabolic trough. The solar receiver can be formed either as a directly absorbing receiver in which a colored water carrier medium flows through the receiver tube allowing the solar radiation to pass therethrough, which medium then directly absorbs the incident solar radiation, or be formed as a volumetric receiver in which a porous absorber is arranged in the interior of the receiver tube and absorbs the incident solar radiation, the heat carrier medium flowing therethrough and being heated convectively. Usually, liquid media, such as water or thermal oil, are used as the heat carrier medium.
U.S. Pat. No. 4,446,851 describes a surface concentrator comprising multiple supply and drain chambers extending longitudinally, being arranged next to one another and being connected to an absorber arranged above the chambers. The heat carrier medium flows through the absorber in a transverse direction, i.e. from a supply chamber to a drain chamber. In the chambers, the heat carrier medium flows in a longitudinal direction. This solar receiver is not adapted to be employed in a trough concentrator, as it comprises too large an area to be arranged in the small concentration area of a trough concentrator. Additionally, the construction of the solar concentrator requires the use of an insulation material which has to be heated whenever the solar thermal power plant starts operating. Especially in the case of trough concentrators where a relatively small concentration factor is obtained, this leads to a noticeable deterioration of efficiency.
In the case of elongated solar receivers for trough concentrators, the heat carrier medium flows through the porous absorber in the longitudinal direction; thus, it has to be transported through the entire length of the absorber. In the case of the usually cylindrical solar receivers, it is an objective to select the cross section thereof to be as small as possible to minimize the surface and thus the loss by dissipation. However, this leads to an increase in the speed of the heat carrier medium resulting in an increased loss of pressure of the heat carrier medium, as the loss of pressure within a porous absorber increases quadratically to the speed of the heat carrier medium. To counteract this loss of pressure, energy for compressing the heat carrier medium has to be employed, which reduces the efficiency of the installation. In the case of usual solar receivers of this kind, a less than optimum compromise has to be achieved between the size of the tube, which should be as large as possible in the area of the incidence of radiation for receiving the radiation, and as small as possible in the area opposite the radiation for reducing the loss due to emission, and the speed of the flow the loss of pressure depends on.
For the pressure not to decrease too far, the heat carrier medium is introduced into the absorber at a high pressure of up to 30 bars. The operation of such installations has shown that occasional leaks cannot be avoided. This led to a number of security and environmental requirements which can makes the construction of such an installation considerably more expensive.